PROJECT SUMMARY (See instructions); Throughout life, the nervous system acquires and maintains many different motor skills. These skills rely on the spinal circuits that determine the interactions between the spinal cord, which excites the muscles, and the peripheral apparatus that produces movements and provides sensory feedback. It is now clear that the brain continually adjusts these circuits to support new skills, to maintain older skills, and to reduce the functional impact of aging, trauma, and disease. The goal of this project is to advance understanding of how the brain exerts the long-term control over these circuits that keeps the spinal cord and the periphery working together effectively, and to apply that understanding to develop new therapeutic methods. Animals and humans can gradually change the spinal stretch reflex (SSR) or its electrical analog, the Hreflex, in response to an operant conditioning protocol. These reflex changes are simple motor skills (i.e.,